The aims of this proposal are to develop the use of catalytic RNAs or ribozymes for therapy. Work will continue on the hammerhead, a well-characterized RNA cleavage reagent, with a focus on understanding why cleavage of a target embedded in a large RNA is often much slower than short oligomers. In addition, experiments are proposed to obtain the elemental rate constants of target binding and cleavage for a hammerhead inside cells in order to better correlate in vitro and in vivo experiments. Finally, the recent discovery that a crosslinked hammerhead is an effective RNA ligase will be studied with the aim of using it as an RNA repair reagent. In an attempt to expand the repertoire of ribozymes that could be used in therapy, two in vitro selection strategies are proposed that are intended to identify novel ribozymes that act on proteins. One selection will search for a dephosphorylase, the other a peptidase.